<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body {
            height: 2000px;
        }
        
        .back {
            position: fixed;
            width: 50px;
            height: 50px;
            background-color: pink;
            right: 30px;
            bottom: 100px;
            display: none;
        }
        
        .container {
            width: 900px;
            height: 500px;
            background-color: skyblue;
            margin: 400px auto;
        }
    </style>
    <script src="jquery.min.js"></script>
</head>

<body>
    <div class="back">返回顶部</div>
    <div class="container">
    </div>
</body>

<script>
    $(function() {
        let container = $('.container');
        let back = $('.back');
        $(window).scroll(function() {
            console.log($(document).scrollTop());
            console.log(container.offset().top);
            if ($(document).scrollTop() >= container.offset().top) {
                back.fadeIn();
            } else {
                back.fadeOut();
            }
        })
        back.click(function() {
            //$(document).scrollTop(0);
            $('body, html').stop().animate({
                scrollTop: 0
            });
        })
    })
</script>

</html>